System for controlling a marine seismic array

ABSTRACT

System for controlling seismic arrays comprising at least one deflector coupled to one side of the towing vessel through at least one wire, lead-in or similar, the detector being positioned at a distance perpendicular to the vessel&#39;s direction of movement, the vessel being provided with a navigation system for measuring the position of the vessel. The vessel comprises calculation means for, on the basis of the vessel&#39;s position, calculating deviations in vessel&#39;s position from a predetermined path. The wire is coupled to the vessel through control organs, e.g. a winch, adapted to vary the wire length from the vessel to the deflector. The control organs are coupled to the calculation means for adjusting the wire length based on the deviations in the position of the towing vessel, thus to avoid corresponding deviations in the movements of the deflector.

[0001] This invention relates to a system and a method for controlling atowed seismic array comprising at least one deflector on one side of atowing vessel coupled through at least one wire, lead-in or similar, thedeflector being positioned with a distance perpendicular to the towingvessels direction of movement, the towing vessel being provided with anavigation system for measuring the position of the vessel.

[0002] In seismic surveys at sea a number of seismic cables are usuallybeing towed after a vessel. The survey of the geological formations atthe sea bottom is performed by sending sound waves from one or moreacoustic sources down into the sea bottom where they are reflected atthe transitions between different types of formations. The reflectedsignals are received by sensors positioned in the seismic cables. Thetowed cable array is towed along a chosen path to perform the survey ina chosen area. The movements of the vessel and the array must becontrolled precisely to secure a coverage of the wanted areas.

[0003] Accurate control of the vessels and arrays positions areespecially important when the same reflection points are to be surveyedmore than once to improve the measurements. This may be done bycontrolling the time between the emitted acoustic signals relative tothe distance between the sensors and the vessels velocity so that thenext signal is reflected from the reflection point up to a later sensoralong the same cable.

[0004] To secure accurate measurements it is usual to monitor theposition of the vessel using existing navigation systems to correctthis. It has, however, become evident that it is difficult to obtain therequired accuracy, among other reasons because of the size and momentumof the vessels. Different systems have also been used for compensatingfor the errors, e.g. signal treatment, over sampling or by controllingthe deflectors pulling the array sideways out from the vessel, thusincreasing the complexity of the system, e.g. as the control signals andpower has to be transmitted from the vessel out to the deflectors.

[0005] U.S. Pat. No. 4,781,140 describes a system for compensating forthe orientation of a vessel relative to the direction of movement. Theseismic cables are coupled to rigid beams the orientation of which arechanged using wires when the vessel for example has to be directedagainst the wind or current. It does, however, not provide anypossibility for compensating for deviations in the vessels position fromthe predetermined path.

[0006] It is an object of the present invention to provide a simplesystem using existing navigation systems for compensating for drift andmovements relative to a predetermined course, without making largedemands to the manoeuvring systems of the vessel and to the systemstreating the seismic data. It is also an object of the present inventionto provide a system using simple and commercially available equipmentposition on the vessel.

[0007] It is a further object of the invention to provide a system and amethod making it possible to maintain the seismic cables in a linearmovement even if the towing vessel must perform manoeuvres departingfrom the predetermined direction within certain limits.

[0008] These objects are obtained using a system and a method asdisclosed in the accompanying independent claims.

[0009] The invention will be described in detail below with reference tothe accompanying drawings, illustrating the invention by way ofexamples.

[0010]FIG. 1 illustrates movements of a seismic survey vessel accordingto the known art.

[0011]FIG. 2 illustrates the movements of a seismic survey vesselaccording to the invention.

[0012]FIG. 1 illustrates how a seismic survey vessel 1 according to theknown art may move under influence of different wind and currentconditions. The vessel 1 is coupled to a seismic array 2, 3, 4, 8comprising towing cables 2, 8 and deflectors 3 stretching a tow sidewaysrelative the towing direction, and seismic cables 4 comprising sensors,such as hydrophones. In addition the tow comprises seismic sources 5,which in this case are coupled directly to the vessel 1.

[0013] In FIG. 1 the vessel is supposed to follow a predetermined course6, but because of wind and current conditions the real movement 7 willdeviate from this. The control system compensating for the deviationswill usually result in an oscillating movement around the predeterminedcourse, which is illustrated in the vessels positions A, B, and C in thedrawing. The seismic array, being coupled to the vessel with towingcables, lead-ins or similar 2, 8 having a fixed length, will follow themovements of the vessel.

[0014] In FIG. 2 the vessel is provided with devices changing the lengthof the towing cables as a response to changes in the position of thevessel relative to the predetermined course 6. These devices may be anykind of available equipment, e.g. winches, and will not be described inany detail here. One example of a possible winch is the Scancontrol 2000system from Scandinavian Control Systems AS, Norway, being mainly usedin fishing trawlers. In some cases already existing winches for takingin or out the towing cables, lead-ins or similar may be used, when theyare suitable for connecting to a control system.

[0015] The vessel is in a usual manner equipped with navigation systems,e.g. the GPS (Global Positioning System), which may provide a measure ofthe deviations from the predetermined course, and which may be used tocompensate for the variations in the movements of the vessel. As it issignificantly easier and faster to pull in or let out the towing wirethe position of the array may be adjusted faster than the vesselsposition. Thus the towed seismic array, represented by the deflectors 3in positions B and C, may keep a more stable course than the towingvessel itself.

[0016] In position B in FIG. 2 the vessel 1 is at the left of thepredetermined course. To compensate for this the towing cable 2 isshortened, thus moving the deflector closer to the vessel. In a similarmanner the right towing cable is let out, so that the right deflectorincreases its distance to the vessel 1.

[0017] In position C in FIG. 2 the vessel is to the right of thepredetermined path, and the towing cables are compensated by making theright cable shorter and the left cable longer.

[0018] For exact control over the seismic arrays position the vessel mayprovided with devices for measuring the positions of one or more of thearray parts, e.g. the deflectors, relative to the vessel. Then themeasured deviation in the vessels position may be compensated directlyby letting out or pulling in a sufficient amount of cable until thedeflector has the correct position, both globally and relative to thevessel.

[0019] Alternatively the cable length being let out or pulled in isadjusted as a function of the angle θ between a line from the vessel tothe deflector and the predetermined direction of movement and thedeviation from the vessels predetermined position.

[0020] In case of deviations being perpendicular to the predeterminedcourse the cable length may be adjusted with${{\Delta \quad k} = \frac{a_{\bot}}{\sin \quad \theta}},$

[0021] Δk being the change in the cable length and a, being the measureddeviation perpendicular to the direction of movement.

[0022] If the position of the vessel deviates in the direction ofmovement, e.g. due to changes in the velocity, this may be compensatedin the same way according to${{\Delta \quad k} = \frac{a_{}}{\cos \quad \theta}},$

[0023] a_(∥) being the deviation parallel to the predetermined course.

[0024] Preferably this compensation is done simultaneously with all thecables, lead-ins etc being coupled to the system. This may of coursealso be done in combination with compensation for the transversaldeviation a⊥.

[0025] In practice these simple models must be adjusted relative to thetowing resistance of the towed seismic array.

[0026] As is evident from FIG. 2 the compensation for the transversaldeviation will lead to a relative displacement of the deflectors in thedirection of movement, thus changing the relative position of theseismic cables. If this makes a problem in the data acquisition thechange may be compensated for in different ways, e.g. by adjusting thelengths of the seismic cables or by using active deflectors increasingtheir lift so that the variation in the used cable length is less whilethe deflector lift capability is used to keep a more even positionrelative to the towing vessel in the direction of movement. One exampleshowing such an active deflector is for example disclosed ininternational patent application No. PCT/NO97/00302.

[0027] For increased precision the heave, roll and pitch movements ofthe vessel may be measured and compensated for according to theinvention.

[0028] In the preferred embodiment of the invention the system comprisesa deflector on each side of the vessel, in which the adjustment at leastcomprises the towing cables, but may also include the rest of the wires,cables, or lead-ins 8 in the shown embodiment and stretching to each ofthe seismic cables 4. This will also provide a possibility for adjustingthe seismic cables position in the direction of movement forcompensating for the displacement which otherwise will follow from thechange in the length of the towing cable 2 to the deflector 3.

[0029] The drawings shows a towed array where the seismic cables 4 areconnected to each other and to the deflectors with cables 9 thusproviding the required distance between the cables. It is of coursepossible to split the tow into a number of parts, each being coupled toone or more deflectors, and where the position of each of these partsmay be adjusted to keep the towed seismic array in a predeterminedcourse.

[0030] Even if the drawings show a symmetrical tow it is of coursepossible to used the invention with asymmetrical arrays, for example incases where a number of vessels together creates a towed seismic array.One solution when the whole array is towed on one side of the vessel mayalso be contemplated, e.g. in surveys close to the shore, where thevessel must be kept at a certain distance from land.

[0031] Even if the invention is primarily described as a means forcompensating for drift relative to the predetermined course it is clearthat it also may be used for keeping the the seismic array along apredetermined path while the vessel for example manoeuvres to avoidobstructions. For example it may be used in rivers where the sailablepath is not necessarily straight.

1. System for controlling a marine seismic array comprising at least one deflector at one side of a towing vessel coupled through a wire, lead-in or similar, the deflector being placed in a position with a distance perpendicular to the direction of movement of the towing vessel, the towing vessel being equipped with a navigation system for measuring the position of the vessel, characterized in that the vessel comprises calculation means for based on the position of the vessel calculating deviations in the movements of the vessel relative to a predetermined course, that the wire is coupled to the towing vessel though control means, e.g. a winch, for varying the wire length from the towing vessel to the deflector, that the control means are coupled to the calculation means for adjusting the wire length depending on said deviations in the vessel position, thus to counteract corresponding deviations in the movements of the deflector.
 2. System according to claim 1, characterized in that a deviation in the vessel position perpendicular to the vessels direction of movement is compensated by the control organ by adjusting the released wire length with a length corresponding to the deviation divided by sinus to the angle between a line from the control organ to the deflector and the planned direction of movement of the towing vessel.
 3. System according to claim 1 or 2, characterized in that a deviation in the vessel position relative to the vessels direction of movement is compensated by the control organ by adjusting the released wire length with a length corresponding to the deviation divided by cosines to the angle between a line from the control organ to the deflector and the planned direction of movement of the towing vessel.
 4. System according to claim 1, characterized in that the towing vessel comprises means for measuring the position of the deflector relative to the vessel, and that the compensation comprises controlling the global position of the deflector based on the position of the towing vessel and the deflectors position relative to the towing vessel.
 5. System according to any one of the preceding claims, characterized in that it also comprises per se known means for measuring deviations in the vessels orientation, such as roll, pitch and heave, and the control means also are adapted to compensate for these movements.
 6. System according to any one of the preceding claims, characterized in that it comprises one deflector on each side relative to the direction of movement of the towing vessel, each deflector being coupled to one control organ and the where the seismic array is spanned between the deflectors.
 7. System according to any one of the preceding claims, characterized in that it also comprises means for adjusting the lift capability of the deflector for adjusting the position of the deflector.
 8. Method for controlling a marine seismic array comprising at least one deflector at one side of a towing vessel coupled through a wire, lead-in or similar, the deflector being placed in a position with a distance perpendicular to the direction of movement of the towing vessel, the towing vessel being equipped with a navigation system for measuring the position of the vessel, characterized in calculating the deviations in the movements of the vessel relative to a predetermined course, varying the wire length from the towing vessel to the deflector depending on said deviations in the vessel position, thus to counteract corresponding deviations in the movements of the deflector.
 9. Method according to claim 8, characterized in that compensation for deviations in the vessel position perpendicular to the vessels direction of movement is performed by varying the released wire length with a length corresponding to the deviation divided by sinus to the angle between a line from the control organ to the deflector and the planned direction of movement of the towing vessel.
 10. Method according to claim 8 or 9, characterized in that compensation for deviations in the vessel position relative to the vessels direction of movement is performed by varying the released wire length with a length corresponding to the deviation divided by cosines to the angle between a line from the control organ to the deflector and the planned direction of movement of the towing vessel.
 11. System according to claim 8, characterized in measuring the position of the deflector relative to the towing vessel, and controlling the global position of the deflector based on the position of the towing vessel and the deflectors position relative to the towing vessel.
 12. System according to any one of claims 8-11, characterized in measuring deviations in the vessels orientation, such as roll, pitch and heave, and compensating for these movements by varying the release wire lengths.
 13. System according to any one of claims 8-12, characterized in adjusting the lift capability of the deflector for adjusting the position of the deflector. 